The present invention relates to a relief valve provided with a pressure increase damping property and adapted to control or regulate hydraulic pressure in a hydraulic circuit of a hydraulic motor for swivelling an operation chamber, for example, of an industrial constructional machine such as power shovel.
Generally, a relief valve is disposed in a hydraulic circuit of a hydraulic motor of an industrial constructional machine such as power shovel, for example, for suppressing the increase of the hydraulic pressure to a predetermined value when the power shovel is accelerated or braked. A relief valve utilized for the purpose described above generally includes a damper piston for eliminating or suppressing shock caused at the time when the power shovel is accelerated or braked, for example, as disclosed in the Japanese Utility Model Publication Nos. 63-19668 (19668/1988) and 63-21814 (21814/1988). In the disclosed technique, the hydraulic pressure in the hydraulic circuit at the piston stroke starting time is considerably lower than that at the relief operation starting time.
The hydraulic circuit of the hydraulic motor is constituted by a cross circuit in which two relief valves are non-parallel connected between a pressurized fluid supply port and a pressurized fluid discharge port of the hydraulic motor, or constituted by an absolute pressure circuit in which inflow sides of two relief valves are respectively connected to the pressurized fluid supply and discharge ports, and escape sides of the relief valves are connected to an external tank. The cross circuit additionally requires an overload relief valve, resulting in a complicated structure and increased cost. For this reason, the absolute pressure circuit has often been utilized for the relief valve of the character described above.
However, since there is some degree of hydraulic pressure in a hydraulic circuit of a hydraulic motor at usual times when there is no accelerating time or braking, when the absolute pressure circuit is utilized, a pressure difference is liable to be caused between the inflow side and the escape side of the relief valve, and may result in the completion of operation of the damper piston before the actual start of operation of the relief valve. In such case, the damper effect cannot be attained at all during actual relief valve operation and large shock is thus caused.
This adverse phenomenon may be eliminated by increasing the initial load of the spring urging the piston. However, this technique increases the starting pressure of the relief operation and accordingly, sufficient damper effect cannot be expected, resulting in the generation of a shock.